1. Field of the Invention
The invention relates to a solar cell with metal/quasi insulator/semiconductor (MIS) or semiconductor/quasi insulator/semiconductor (SIS) layer arrangement where the quasi insulating layer consists of a high resistance semiconductor material.
2. Prior Art
MIS and SIS solar cells are known. In principle, their function is similar to that of a Schottky diode (metal/semiconductor contact); their photovoltaic characteristics are, however, improved by the addition of a extremely thin insulating film. In general, the insulating film consists of a natural oxide of the semiconductor. The thickness of the insulating film is chosen in a way that the additional potential barrier caused by it hinders the minority carrier flow from the semiconductor into the metal. On the other side, however, the minority carriers must still be able to cross the additional barrier without impairing the fill factor and the short-circuit current of the solar cells. This requires a technologically extensive, very precise and regular control and adjustment of the thickness of the insulating film at a very low level of around 2 nm.
Because of reproducibility, simplicity and time savings factors, it is desirable to use a physically deposited heterogeneous insulating material. Because of the extremely low thickness of the insulating layer, however, variations of the film thickness of pinholes (microscopic holes) within the insulating film may be caused by a relatively small roughness on the surface of the photoelectrically active semiconductor and/or by extremely small dust particles. They have a very negative effect on the photovoltaic characteristics of the SIS or MIS solar cells, and special protective measures are required. It is, therefore, appropriate to use thicker insulating films which, however, will not impair the fill factor and the short circuit current of the solar cells when under the impact of light.
It is known that the function of an insulator in SIS and MIS solar cells can also be fulfilled by a semiconductor with a sufficiently large energy gap so that a contact/semiconductor/quasi insulating semiconductor/basis semiconductor structive is formed. In this way, e.g. Au/ZnSe/CdSe thin layer solar cells have already been described (D. Bonnet and E. Rickus, The CdSe Thin Film Solar Cell, Proc. 14th IEEE Photovolt. Specialists Conf., 1980, page 629). The use of an approx. 3.5 nm thick ZnSe film as a quasi insulator semiconductor results when under the impact of light, in short circuit current densities of approx. 18 mA/cm.sup.2 and in fill factors of approx. 55%.
The relatively low open circuit voltage of approx. 0.4 V is caused by partial short circuits due to surface roughness or pinholes between the cadmium selenide and the Schottky contact. An increase of the thickness of the zinc selenide film to approx. 5 nm increases the open circuit voltage to values of around 0.6 V. While the short circuit current remains practically unchanged, the fill factor drops to values about 45%.